Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface showing our new design.

The even length dashed broken lines are for environmental purposes only and form no part of the claimed design.

The uneven length dash-dot broken lines of the display screen represent an unclaimed boundary line and form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 